Power and thermal management are becoming more challenging than ever before in all segments of computer-based systems. While in the server domain, the cost of electricity drives the need for low power systems, in mobile systems battery life and thermal limitations make these issues relevant. Optimizing a system for maximum performance at minimum power consumption is usually done using the operating system (OS) to control hardware elements. Most modern OS's use the Advanced Configuration and Power Interface (ACPI) standard, e.g., Rev. 3.0b, published Oct. 10, 2006, for optimizing the system in these areas. An ACPI implementation allows a core to be in different power-saving states (also termed low power or idle states) generally referred to as so-called C1 to Cn states.
When the core is active, it runs at a so-called C0 state, but when the core is idle, the OS tries to maintain a balance between the amount of power it can save and the overhead of entering and exiting to/from a given state. Thus C1 represents the low power state that has the least power savings but can be switched on and off almost immediately, while extended deep-low power states (e.g., C3) represent a power state where the static power consumption is negligible, but the time to enter into this state and respond to activity (i.e., back to C0) is quite long. Note that different processors may include differing numbers of core C-states, each mapping to one ACPI C-state. Multiple core C-states can map to the same ACPI C-state.
OS C-state policy has a number of drawbacks. First, it selects C-state based on historical central processing unit (CPU) utilization (i.e., C0 residency time). For a dynamic workload this decision is often wrong, resulting in either less power savings or large performance losses caused by wrongfully entering deep sleep states with long entry/exit latencies. Second, CPU utilization is sampled at a coarse granularity, e.g., 100 milliseconds (ms). Some transient opportunities such as an idle state lasting for hundreds of microseconds (μs) could be missed. For example, if the last 100 ms CPU utilization is 85%, the OS will use the C1 state. At this utilization level, dynamic server workloads still have many 100-500 μs long idle periods. Third, the policy does not consider activities of other cores in the same package. Since server workloads are typically multi-tasked and each task is short, if one core is in a deep sleep state and unable to service a task in time, other cores with a lighter load may be able to accommodate these tasks. Current approaches thus fail to extract additional power savings.